Jochen Lach

Triazolopyridines as ligands: structural diversity in iron(II), cobalt(II), nickel(II) and copper(II) complexes of 3-(2-pyridyl)-[1,2,4]triazolo[4,3-a]pyridine (L10) and spin crossover in [FeII(L10)2(NCS)2]

The coordination chemistry of the ligand 3-(2-pyridyl)-[1,2,4]triazolo[4,3-a]pyridine (L¹⁰) has been investigated and iron(II), cobalt(II), nickel(II) and copper(II) complexes featuring diverse structural motifs have been prepared. In the 2 : 1-type complexes [Co(II)(L¹⁰)₂(MeOH)₂](ClO₄)₂ (20), [Ni(II)(L¹⁰)₂(MeOH)₂](ClO₄)₂ (21), [Cu(II)(L¹⁰)₂(MeOH)₂](ClO₄)₂ (22), [Co(II)(L¹⁰)₂(H₂O)₂](ClO₄)₂ (23) and [Cu(II)(L¹⁰)₂(ClO₄)₂] (24) the metal centres are N₄O₂ octahedrally coordinated with two N²,N(pyr) bidentate ligands L¹⁰ in the equatorial positions. In the N₆ octahedral 4 : 1-type complex [Co(II)(L¹⁰)₄](ClO₄)₂·H₂O (25) both axially coordinating N¹ unidentate and equatorially bound N²,N(pyr) bidentate ligands L¹⁰ are observed. The N₆ octahedral 3 : 1-type complex [Fe(II)(L¹⁰)₃](OTf)₂·1.5MeCN·0.13H₂O·0.87MeO(t)Bu (27) features three N²,N(pyr) bidentate ligands L¹⁰ in the mer configuration. The two closely related N₆ octahedral complexes [Fe(II)(L¹⁰)₂(NCS)₂] (29) and [Fe(II)(L¹⁰)₂(dca)₂] (30) have fundamentally different structures. While complex 29 features two equatorially bound N²,N(pyr) bidentate ligands L¹⁰ and axial NCS⁻ co-ligands, complex 30 is a one-dimensional doubly μ1,5-dicyanamido-bridged polymer with N¹ unidentate ligands L¹⁰ in the axial positions. Temperature-dependent magnetic susceptibility measurements of the iron(II) complexes 28 and 29 have shown the 3 : 1-type complex [Fe(II)(L¹⁰)₃](ClO₄)₂·H₂O (28) to be in the low-spin state over the range 300-2 K and the 2 : 1-type complex 29 to be a spin crossover compound with T(1/2) = 269 K whereas the dicyanamido-bridged complex 30 remains in the high-spin state even down to 113 K, according to X-ray diffraction data. A single end-to-end bridging NCS⁻ co-ligand is found in the N₄S square-pyramidal complex [Cu(II)(L¹⁰)(NCS)₂] (31) which shows Curie-Weiss behaviour over the range 300-2 K. A brief review of the coordination chemistry of triazolopyridines is given.

Complexation, computational, magnetic, and structural studies of the maillard reaction product isomaltol including investigation of an uncommon π interaction with copper(II)

The metal complexation properties of the naturally occurring Maillard reaction product isomaltol HL(2) are investigated by measurement of its stability constants with copper(II), zinc(II), and iron(III) using potentiometric pH titrations in water, by structural and magnetic characterization of its crystalline complex, [Cu(L(2))(2)]·8H(2)O, and by density functional theory calculations. Strong complexation is observed to form the bis(isomaltolato)copper(II) complex incorporating copper in a typical (pseudo-)square-planar geometry. In the solid state, extensive intra- and intermolecular hydrogen bonding involving all three oxygen functions per ligand assembles the complexes into ribbons that interact to form two-dimensional arrays; further hydrogen bonds and π interactions between the furan moiety of the anionic ligands and adjacent copper(II) centers connect the complexes in the third dimension, leading to a compact polymeric three-dimensional (3D) arrangement. The latter interactions involving copper(II), which represent an underappreciated aspect of copper(II) chemistry, are compared to similar interactions present in other copper(II) 3D structures showing interactions with benzene molecules; the results indicate that dispersion forces dominate in the π system to chelated copper(II) ion interactions.

Stabilization of hypophosphite in the binding pocket of a dinuclear macrocyclic complex: synthesis, structure, and properties of [Ni(2)L(μ-O(2)PH(2))]BPh(4) (L = N(6)S(2) donor ligand).

The dinickel(II) complex [Ni2L(ClO4)]ClO4 (1), where L(2-) represents a 24-membered macrocyclic hexaamine-dithiophenolate ligand, reacts with [nBu4N]H2PO2 to form the hypophosphito-bridged complex [Ni2L(μ-O2PH2)](+), which can be isolated as an air-stable perchlorate [Ni2L(μ-O2PH2)]ClO4 (2) or tetraphenylborate [Ni2L(μ-O2PH2)]BPh4 (3) salt. 3·MeCN crystallizes in the triclinic space group P1̅. The bisoctahedral [Ni2L(μ-O2PH2)](+) cation has a N3Ni(μ1,3-O2PH2)(μ-S)2NiN3 core structure with the hypophosphito ligand attached to the two Ni(II) ions in a μ1,3-bridging mode. The hypophosphito ligand is readily replaced by carboxylates, in agreement with a higher affinity of the [Ni2L](2+) dication for more basic oxoanions. Treatment of [Ni2L(μ-O2PH2)]ClO4 with H2O2 or MCPBA results in the oxidation of the bridging thiolato to sulfonato groups. The hypophosphito group is not oxidized under these conditions due to the steric protection offered by the supporting ligand. An analysis of the temperature-dependent magnetic susceptibility data for 3 reveals the presence of ferromagnetic exchange interactions between the Ni(ii) (S = 1) ions with a value for the magnetic exchange coupling constant J of +22 cm(-1) (H = -2JS1S2). These results are additionally supported by DFT (density functional theory) calculations.

STM Study of Au(111) Surface-Grafted Paramagnetic Macrocyclic Complexes [Ni2L(Hmba)]+ via Ambidentate Coligands

Molecular anchoring and electronic properties of macrocyclic complexes fixed on gold surfaces have been investigated mainly by using scanning tunnelling microscopy (STM) and complemented with X-ray photoelectron spectroscopy (XPS). Exchange-coupled macrocyclic complexes [Ni2L(Hmba)](+) were deposited via 4-mercaptobenzoate ligands on the surface of a Au(111) single crystal from a mM solution of the perchlorate salt [Ni2L(Hmba)]ClO4 in dichloromethane. The combined results from STM and XPS show the formation of large monolayers anchored via Au-S bonds with a height of about 1.5 nm. Two apparent granular structures are visible: one related to the dinickel molecular complexes (cationic structures) and a second one related to the counterions ClO4(-) which stabilize the monolayer. No type of short and long-range order is observed. STM tip-interaction with the monolayer reveals higher degradation after 8 h of measurement. Spectroscopy measurements suggest a gap of about 2.5 eV between HOMO and LUMO of the cationic structures and smaller gap in the areas related to the anionic structures.

Chemisorption of Exchange‐Coupled [Ni2L(dppba)]+ Complexes on Gold by Using Ambidentate 4‐(Diphenylphosphino)benzoate Co‐Ligands

A new strategy for the fixation of redox-active dinickel(II) complexes with high-spin ground states to gold surfaces was developed. The dinickel(II) complex [Ni2L(Cl)]ClO4 (1ClO4), in which L(2-) represents a 24-membered macrocyclic hexaaza-dithiophenolate ligand, reacts with ambidentate 4-(diphenylphosphino)benzoate (dppba) to form the carboxylato-bridged complex [Ni2L(dppba)](+), which can be isolated as an air-stable perchlorate [Ni2L(dppba)]ClO4 (2ClO4) or tetraphenylborate [Ni2L(dppba)]BPh4 (2BPh4) salt. The auration of 2ClO4 was probed on a molecular level, by reaction with AuCl, which leads to the monoaurated Ni(II)2Au(I) complex [Ni(II)2L(dppba)Au(I)Cl]ClO4 (3ClO4). Metathesis of 3ClO4 with NaBPh4 produces [Ni(II)2L(dppba)Au(I)Ph]BPh4 (4BPh4), in which the Cl(-) is replaced by a Ph(-) group. The complexes were fully characterized by ESI mass spectrometry, IR and UV/Vis spectroscopy, X-ray crystallography (2BPh4 and 4BPh4), cyclic voltammetry, SQUID magnetometry and HF-ESR spectroscopy. Temperature-dependent magnetic susceptibility measurements reveal a ferromagnetic coupling J = +15.9 and +17.9 cm(-1) between the two Ni(II) ions in 2ClO4 and 4BPh4 (H = -2 JS1S2). HF-ESR measurements yield a negative axial magnetic anisotropy (D<0), which implies a bistable (easy axis) magnetic ground state. The binding of the [Ni2L(dppba)]ClO4 complex to gold was ascertained by four complementary surface analytical methods: contact angle measurements, atomic-force microscopy, X-ray photoelectron spectroscopy, and spectroscopic ellipsometry. The results indicate that the complexes are attached to the Au surface through coordinative Au-P bonds in a monolayer.